1. Field of the Invention
The present invention relates to a hierarchical controls system architecture and method of controlling an image output by a marking engine of an image output terminal (IOT). The hierarchial architecture and associated method for controlling image output can be easily implemented in a wide variety of IOTs, especially in next generation marking engines.
2. Description of the Related Art
The controls system architecture for IOTs that output only black images (non-color IOTs) provide good IOT design using comparatively simple process control loops. While the methods and techniques developed for the marking engines of black only IOTs are successful, they cannot be used effectively with the marking engines of color IOTs. Color IOTs are required to produce high quality pictorial color images using several different primary colors. The process controls required for the color marking engines need to be significantly more complex as compared to the process controls for black only marking engines. Furthermore, as the popularity of color IOTs has increased rapidly, there is also greater customer demand for color quality and more stable and predictable output of the color IOT. Since precise control of multiple parameters is required to stabilize color IOTs, the available process data needs to be used effectively. The control of the color IOTs is complicated by the fact that multiple development subsystems are used in the color printing process rather than only one development subsystem used in black only IOTs. Moreover, if a tandem engine is used, control of four complete xerographic modules is required.
Thus, the traditional difficulties associated with the control of color IOTs along with the public demand for increased IOT image quality and reliability require a more complex controls system and a more comprehensive design for the controls system.
In digital laser printing and reproducing operations, a continuous tone image cannot be identically reproduced because of the limitations of the laser in the printer. The laser cannot identically match the continuous tone image because there are two sources which limit the ability of the IOT to write the continuous tone images on the photoreceptor. The first source of limitation is the difficulty in reliably determining which value of intensity of laser power should be used to partially expose the photoreceptor at a specific location at a given point in time. The photoreceptor inherently has non-uniformities which occur in space and time rendering this problem intractable. The second source of limitation is that the development process produces more stable image quality when the laser is operated to be either on or off and controlled such that a dot is either printed or not printed. To overcome these two limitations, a continuous tone image to be printed or reproduced must be interpreted such that the IOT mimics the continuous and gray tones of the image as accurately as possible. This is done by the known process of halftoning which involves filling in a certain percentage of each of the halftone cells to most accurately match the continuous tone image. The method used to reproduce a continuous tone image accurately is to match a tone reproduction curve of the continuous tone image as closely as possible. A tone reproduction curve is a series of assigned setpoints representing different half tone cell densities ranging from white (no coverage) to full solid area coverage. This curve can be assigned a certain number of levels of halftone cell densities. Each level or point on the TRC represents a different halftone cell density. When an IOT receives data from a continuous tone image to be printed or reproduced, these data correspond to a desired darkness at a location on the output print. The data are assigned to a location on the TRC corresponding to a density value near the desired darkness. So while the laser is writing individual dots in either black or white, the number and arrangement of these dots is chosen so that, at a distance, the appearance of tonal gradations is achieved. The greater the number of different tones or shades of gray that can be printed, the more closely the printer or copier can mimic the actual continuous tone image.
To achieve a high quality image, the tone reproduction curve of the image to be printed or copied must be maintained by the controls system of the IOT during the entire printing or copying process. A tone reproduction curve (TRC) of the image output by the printer or copier is affected by several variables, including changes in environmental factors such as humidity and temperature and uncontrolled changes in the xerographic elements, such as the photoreceptor, laser and developer material. All of these factors can cause the tone reproduction curve to vary, thereby altering the appearance of the output image. The control systems of the prior art have concentrated on correcting variances in the tone reproduction curve by separately controlling a few particular factors affecting the tone reproduction curve. The problem with controlling only a few factors separately is that the tone reproduction curve is affected by interactions involving a plurality of factors. Changing one factor to correct a variance in the tone reproduction curve may cause another factor to vary the tone reproduction curve and thus, the desired tone reproduction curve is not accurately maintained.